This invention relates generally to radio frequency circuits and, more particularly, to radio frequency circuits which provide a selected phase shift to a signal propagating therethrough.
As it is known in the art, phase shifter circuits are employed in various radio frequency (RF) applications such as phased array antenna systems and phase modulators. One type of phase shifter includes a first plurality of passive elements which provide a phase lag or negative phase shift network and a second, different plurality of passive elements which provide a phase lead or positive phase shift network. The phase lag and phase lead networks are configured between a pair of terminals of the phase shifter and with a pair of switches disposed to alternatively couple one of the networks to the pair of terminals at any time. Typically, each of the pair of switches is further provided by a pair of MESFETs or other suitable microwave switching devices. The MESFETs, for example, are fed complimentary control signals to select between one of the two transistors and hence one of two signal paths through either the phase lag or phase lead network. Thus, in accordance with the control signals fed to the transistors pair of switches, either the phase lag or the phase lead network is switched into the signal path to thus provide to a signal propagating therethrough a phase lag or a phase lead. The field effect transistors (such as MESFETs) are employed in these applications because they are easily formed as part of monolithic microwave integrated circuits unlike other types of active switching devices such as pin diodes.
A problem with this approach, however, is that since the filtering networks are provided using lumped elements such as inductors and capacitors, the circuits can be relatively large. This is particularly true at low microwave frequencies, where the elements, particularly the inductive elements, are very large and thus circuits fabricated with these devices are, likewise relatively large. This is undesirable for monolithic microwave integrated circuits from considerations of packaging, as well as cost. Moreover, the conventional approach requiring the use of a pair of input FETs and a pair of output FETs to provide the input and output switching likewise requires the use of complimentary control signals to switch the FETs. In applications of such phase shifters such as phased arrays and phase modulators, the above switching requires additional driving circuits to produce a complimentary control signal and, moreover, requires an additional control signal conductor to carry a complimentary signal to each one of the phase shifting elements.